1. Field of the Invention
This invention relates generally to injection molding. In particular, this invention relates to nozzles for controlling flow of melt through conduits between an injection unit and mold cavities.
2. Description of Related Art
Injection molding is a cyclic process wherein flowable material, e.g. molten metal, rubber or thermoplastic (known as “melt”) is forced into mold cavities having the shape and size of articles to be molded and allowed to solidify; the molded articles are removed from the mold cavities and the process is repeated. Mold assemblies defining mold cavities are operable between “open” and “dosed” conditions, the open condition permitting release of molded articles and the closed condition permitting filling of the cavities with the material to be molded. Once filled, the mold assembly is held closed while the molten material solidifies and is opened to allow removal of molded articles when the material has sufficiently solidified to retain the molded form without unacceptable distortion when the material is unsupported. Mating mold components of a mold assembly comprise a primary core component and primary cavity component, the primary core component and primary cavity component meeting along a so called “parting line” when the mold assembly is closed. A mold assembly within the press unit of a molding machine is illustrated in FIG. 1a. Filling of cavities is effected by forcing melt from an injection unit through conduits to the mold cavities. It is known in injection molding to provide valves for controlling flow of melt from the injection unit to the mold assembly.
It is known that each pair of mating mold components may define plural mold cavities. For such mold assemblies, it is known to provide conduits for conducting melt from the injection unit to at least one opening at each cavity, the opening referred to as a “gate”. The conduits may comprise channels open to the parting plane of the mold components and known as sprue channels or manifolds assembled with the cavity defining members and connected to the gates by nozzles. It is known to provide a manifold extension nozzle or sprue bushing between the sprue channel or mold manifold and the injection unit nozzle, the manifold extension nozzle or sprue bushing having an inlet conforming to the shape and size of the injection unit nozzle and an outlet in sealing engagement with the sprue channel or manifold.
Improved productivity of injection molding is achieved by providing mold arrangements comprising plural mold assemblies having plural parting lines wherein the mating mold components are carried on intermediate movable platens interposed between a stationary platen and a primary movable platen defining a press or clamping unit of the injection molding machine. Such arrangements are known as “stack molds” or “dual molds” and FIG. 1b illustrates such a mold arrangement in the press unit of a molding machine. In such mold arrangements, melt is conveyed from the injection unit to the intermediate platens through conduits known as sprue bars and from the intermediate platens to the mold cavities. The intermediate platens are provided with manifolds and extension nozzles to convey melt to the cavity defining members of the mold assemblies. Each manifold is supported in a cavity of the intermediate platen so as to minimize contact between the manifold and platen, creating a so-called “floating manifold”. The floating manifold arrangement reduces heat transfer between the manifold and intermediate platen to improve control of the temperature of melt within the floating manifold. Melt is conducted from the intermediate platen manifold to each mold assembly manifold extension nozzle or sprue bushing by an intermediate nozzle. Dimensions of the floating manifold and intermediate nozzle are subject to change with temperature. To insure integrity of the flow path from the floating manifold to the mold assembly, it is known to select a length for the intermediate nozzle that will maintain sufficient contact between the intermediate nozzle, the floating manifold and the manifold extension nozzle or sprue bushing to resist leakage of melt over a contemplated operating temperature range. In the event that actual operating temperatures fall outside the contemplated range, dimensional changes with temperature may result in leaks of melt or damage to components from excessive compressive forces. Hence there is a need for an intermediate nozzle that maintains flow path integrity over a broad temperature range without producing damaging compressive forces.